The present invention relates generally to means for collimating, aberration correcting, and angularly aligning the output of a diode laser array, and more particularly to a combination of a cylindrical lens and a pair of binary optical elements which are optimized to collimate, aberration correct, and align the individual diodes of a diode laser array such that each individual diode fills its aperture.
Many optical sources such as diode lasers produce asymmetrically divergent beams. Diode lasers are currently utilized in many applications. Many of these applications require collimated diode laser outputs with low optical aberrations even though diode lasers typically produce asymmetrically diverging output beams. In many instances this asymmetric divergence may be quite fast, such as an 80 degree output fan along the fast axis and a 20.degree. output fan along the slow axis of an individual diode laser. These applications also typically require the higher power produced by a diode laser array which for most efficient operation requires the output of the individual diode lasers to fill its aperture such that a planar wavefront of fairly constant intensity is produced instead of an array of individual pinpoints of light wherein each pinpoint emanates from an individual diode laser which has not filled its aperture.
There have been a variety of proposed optical design solutions to collimate with low optical aberrations the asymmetrically diverging wavefront's of diode lasers. One proposed solution utilizes conventional cylindrical lenses to collimate each axis, the fast and the slow axes, independently. If such lenses were used with a rectangular laser diode array each cylindrical lens would collimate a row or a column of laser diode sources. However, the performance of such conventional cylindrical lenses is greatly reduced once the slow axis divergence exceeds approximately a 7 degree full angle measured at the position on the Gaussian output distribution of the laser diode which is 10% of the maximum output value. This reduction in performance is due to skew rays which cannot properly be collimated by either the fast or the slow axes of crossed cylindrical lenses.
Additionally, for more efficient collimation of the fast axis, acylindrical lenses must be fabricated to eliminate spherical aberration. These small cylinders, however, are difficult to produce accurately and affordably.
Another approach for producing an optical element for collimating a divergent output is to mold plastic or glass aspheric optical elements in the shape desired in order to collimate the divergent output beam. An example of such a molded glass optical element is discussed in "Precision Molded-Glass Optics," written by R. Maschmeyer, et al. in Applied Optics, Volume 22, No. 6, on page 2410 in 1983. The glass optic elements which have been molded are generally limited in their ability to collimate a divergent output beam, however, due to stresses inherent in the glass element from the molding process. Additionally, molded plastic optical elements, while easier to mold than the relatively difficult to mold glass elements, tend to deform when subjected to high temperatures. Such deformation limits the plastic elements ability to collimate diode laser's output since high temperatures would often be experienced when the optical element was placed adjacent to the emitting facet of the diode laser.
An additional problem with compact two dimensional laser diode arrays which are arranged in substantially linear columns and rows of laser diodes, which is not addressed by the aforementioned collimation optical elements, is the exhibition of nonlinear bowing of the linear rays such that the diodes are no longer horizontally aligned. The nonlinear bowing of the laser diode array introduces an angular misalignment of the individual diodes upon collimation.
The angular misalignment is due to differences in the vertical position of the output beams of the individual laser diodes as they are being collimated. For example, a linear laser array with a substantially horizontal output that does not suffer from nonlinear bowing will produce a series of output beams that will propagate in the substantially horizontal plane positioned in the plane in which the active layer of the diode array exists. In contrast, laser diodes within the same exemplary laser array that are bowed or misaligned will produce output beams that will be positioned above or below the horizontal plane.
These directional errors of a non-linear "bowed" diode laser array may be corrected by individually fabricated microprism arrays in which individual prisms are placed in each diode's path. Each prism must then be positioned such that there is no angular misalignment upon collimation. The use of microprism arrays in this application is extremely labor intensive and therefore cost prohibitive. Microprism arrays also tend to be fragile and to delaminate with time and temperature cycling. Additionally, microprism arrays do not collimate or aberration correct the diode laser's output as required in numerous applications.
Therefore, it would be desirable to provide a combination of optical elements which are capable of collimating, with few optical aberrations, a highly divergent wavefront. Furthermore, it would be desirable if such collimating optical element would fill the aperture of each individual diode laser within a diode laser array so as to produce a substantially planar wavefront. It would also be desirable to develop an optical element for use in conjunction with the collimating and aberration correcting optical element to correct diode laser arrays which exhibit nonlinear bowing so that the resulting output would be angularly aligned with respect to the plane in which the active region of the individual diode laser lies.